Firstborn Japanese macaque (Macacafuscata) infants are more responsive to novel stimuli than higher birth order infants

There has been increasing interest in the effects of birth order on behavioral development. For example, firstborn human individuals have been reported to be more anxious, as well as more intellectually motivated, compared to those with higher birth order. However, results among studies are discordant, with some finding firstborn individuals to be less anxious than later-born individuals. In this study, we examined the relationship between birth order and responsiveness to novel stimuli in 68 infant Japanese macaques living in a semi-natural 2-acre outdoor corral. We measured responsiveness to novelty using 3 tests: 1) a Freeplay test (measured response to a novel environment with toys), 2) a Remote-Controlled Car test (measured reaction to a potentially threatening nonsocial stimulus, a car driven towards the infant), and 3) a Fruit test (measured response to a novel fruit introduced into the cage). The firstborn infants (n = 13) showed a stronger response to the car than later-born infants (n = 55; P = 0.04). Somewhat surprisingly, the firstborn infants also inspected the novel food significantly sooner than later-born infants (P = 0.02). There were no differences in the Playroom test. In summary, we found that firstborn infants showed a stronger response than later-born infants to novel stimuli. The factors underlying this difference remain to be determined.